FB Messenger Notification Disappearing

I've been worried about being hacked lately, and yesterday I was sent an FB message by a friend, the phone was turned on but tapped off, and then right when I tapped it on, the notification disappeared. Then I swiped in, and saw the bubble had no notification/(1) on it either; the message was still there but it was as though it was read without me doing anything, and nothing but my phone was logged into messenger/facebook according to the security page. My phone was low on battery and died soon after, but I'm still worried someone have hacked my phone. Has something like this ever happened to anyone else or should I be somewhat worried?